Bug Description

Sycndaemon is executing GetDelta requests on the same volume in parallel, when it should only execute 1.
On solution is to add a path_lock based on volume path or id to queue the GetDetla's for the same volume and only execute one at a time (following GetDeltas will not be executed as one is already queued/running)

[Test Case]
Without this fix, excessive GetDelta calls for the same Volume ID will appear in the syncdaemon.log under ~/.cache/ubuntuone/log/. With the fix, after
restarting syncdaemon, whenever a GetDelta call is made for a volume, it will occur only once for each volume.

Hello! This bug is attached to an upload in the queue for precise-proposed. However, it does not contain the requisite fields for the SRU team to process the upload. Please add them per the process outlined here. To be more succinct, make sure the bug description lists these fields: "Impact, Dev Fix, Stable Fix, Regression Potential, Test case".

Accepted ubuntuone-client into precise-proposed. The package will build now and be available in a few hours. Please test and give feedback here. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you in advance!

Tested by restarting syncdaemon with and without changes to volumes between. Observed none of the unwanted operation described in

[Test Case]
Without this fix, excessive GetDelta calls for the same Volume ID will appear in the syncdaemon.log under ~/.cache/ubuntuone/log/. With the fix, after
restarting syncdaemon, whenever a GetDelta call is made for a volume, it will occur only once for each volume.

However, holding off on setting verification-done due to NACK from mdeslaur.